1. HIV integrase We have been engaged in a structural analysis of the HIV integrase, and have previously reported the crystallization and structure determination of the catalytic core domain, residues 50-212. We have now crystallized this core domain in several different crystal forms. Analysis of these crystals reveals more details about some of the residues that were disordered in the original structure. Crystals prepared in the presence of Mg++, a known cofactor, now show clear evidence for the binding of this cation at the active site of the enzyme. We have also prepared a variety of other fragments of the integrase in an effort to crystallize the core domain coupled to either or both of the remaining domains. Some encouraging results have been obtained but diffraction quality crystals have not yet been produced. 2. The Barrier to Autointegration Factor (BAF) BAF is a host factor that prevents the autointegration of the viral DNA. The molecule has been crystallized and the structure determined at 1.9A resolution. In the crystal the protein exists as a dimer. Comparison with previous NMR studies of this protein shows that there is close homology between the structures of the subunit monomers together with a small difference in forming the dimer.